The present invention relates generally to static power conversion systems and their method of operation, and more particularly to a method for improving the response of a converter system having neutral thyristors.
Power conversion systems and their method of operation for supplying a d.c. load from an a.c. source is well known. It is also known that these systems are adapted to control the magnitude of the output d.c. voltage and have the ability to operate in more than one quadrant, a six pulse bridge converter being a typical example. Such a circuit, however, is characterized by a relatively high ripple content in the load voltage at low output voltage. The power factor of this arrangement is also known to be relatively poor at lower output voltages.
When the load is inductive and the load current tends to be continuous, as is the case for a motor load, it is also known that the addition of a free-wheeling diode configuration will reduce load voltage ripple and to improve the power factor at lower output voltages. Such converters as these, however, do not permit fourth quadrant operation and the degree of control remains limited. These principles as well as others relating to known a.c. to d.c. power conversion systems are set forth in detail in Thyristor Phase-Controlled Converters and Cycloconverters, by B. R. Pelly, Copyright 1971 by John Wiley and Sons, Inc. (Library of Congress Catalogue No. 70-125 276) and reference to this work can be made when desirable for a more exhaustive treatment. It is also known that the input power factor of a six pulse phase controlled thyristor bridge converter supplying an inductive load can be improved at low voltage operation by adding two additional thyristors which are connected from the positive and negative bus connected across the load to the neutral point of the a.c. system. Such a configuration as well as the basic circuit concepts are set forth in the now abandoned U.S. patent application, U.S. Ser. No. 748,281, now abandoned, entitled "Power Conversion System and Method of Control", filed on Dec. 7, 1976, by the assignee of the present invention, in the name of David L. LaFuze, which application is specifically incorporated herein by reference.
As will be shown hereinafter, when reference to the drawings in this application is made, when a bridge converter such as disclosed in the LaFuze application, Ser. No. 748,281 is operating at a low d.c. output voltage and with continuous current in the first quadrant, there are intervals in the cycle when the respective thyristors in both neutral legs are in conduction. Flat spots indicative of zero output voltage appear on the d.c. output voltage waveform since the a.c. power system is being by-passed in this operating mode and as a result a transport lag or time delay will exist during voltage reversal as the system makes a transition from first to fourth quadrant operation.